fMRIPrep doesn’t produce standalone fsLR surfaces, and instead creates the full CIFTI-2 file in fsLR space. We use den-91k to denote that the output is in the 91282 Greyordinates space - since this file includes surface vertices (left/right cortex) as well as subcortical volumetric voxels.
If you wish to extract just the surface information, you can use the Connectome Workbench. I think the command to do it would be:
wb_command -cifti-separate <cifti-file> COLUMN -metric CORTEX_LEFT <output-gifti0> -metric CORTEX_RIGHT <output-gifti1>